It is known that the petroleum industry routinely makes use of cracking processes in which hydrocarbon molecules of high molecular weight and with a high boiling point are broken down into smaller molecules boiling at lower temperature ranges appropriate to the desired use.
The process most widely used today for this purpose is the so-called Fluid Catalytic Cracking, or FCC, process. In this type of process, the hydrocarbon feedstock is simultaneously vaporized and contacted at high temperature with a cracking catalyst that is maintained in suspension in the feedstock vapors. After the desired range of molecular weights has been attained by cracking, along with a corresponding lowering of the boiling points, the catalyst is separated from the products obtained.
In processes of this type, the desired lowering of the boiling points results from controlled catalytic and thermal reactions. These reactions occur almost instantaneously when the finely atomized feedstock contacts the catalyst. However, the catalyst is rapidly deactivated during the short time that it is in contact with the feedstock, largely because of the adsorption of hydrocarbons and the deposition of coke on its active sites. It is necessary to separate as rapidly as possible the effluent hydrocarbons from the coke-laden catalyst particles, to strip the latter continuously with steam, for example, in order to recover the adsorbed hydrocarbons, and to reactivate the catalyst particles, also continuously, without altering their characteristics, by proceeding with controlled combustion of the coke in a single- or multistage regeneration section before the catalyst is recycled to the reaction zone.
In practice, the catalyst of the FCC process and the feedstock to be treated are injected under pressure and at elevated temperature at the base of a column known as riser. At the top of the column there is generally a chamber that is concentric with the riser. Disposed in this chamber and above the riser is a ballistic separator which separates the coke-laden catalyst particles from the hydrocarbon vapors. The latter are passed to a fractionator after the catalyst fines have been recovered in a cyclone. The coke-laden catalyst particles fall to the bottom of the separator chamber by gravity. These particles are steam-stripped to recover the hydrocarbons still present in their pores and are then sent to a regenerator in which their catalytic activity is restored by burning off the coke deposited on them during the cracking reaction.
The FCC process is therefore carried out in such a way that the cracking unit is in thermal equilibrium, all necessary heat being supplied by the combustion of the coke deposited on the catalyst particles during the cracking reaction.
The cracking reactions of the feedstock in contact with the catalyst are very rapid and last less than a second, usually about half a second.
It is therefore important carefully to optimize the retention time of the feedstock in the reaction zone during which it is in contact with the catalyst particles. If that time is too long, too much coke, hydrogen and dry gases (ethane and propane) will form, at the expense of gasoline; and if the cracking time is too short, the gasoline yield will be insufficient.
Based on the studies which applicants and/or their assignee have conducted, it has been found that it is desirable to separate the hydrocarbon vapors and the particles of spent catalyst as quickly as possible in the separation chamber in order to prevent the hydrocarbons from being entrained by these particles and from continuing to crack thermally on contact.
Moreover, it is desirable to equalize the retention times of the hydrocarbons in the separation chamber, reducing them to the minimum. The applicants have discovered that in practice these retention times may be as much as double the normal times because of recirculating streams, both above and below the ballistic separator. In fact, hydrocarbon vapor streams form not only above the ballistic separator, between it and the upper part of the separation chamber, but also below the separator, between it and the fluidized bed of coke-laden catalyst particles being stripped. The retention times of the vapors of these diverse streams can differ considerably, and there is the risk of re-entrainment into the fluidized bed of catalyst particles of a portion of the hydrocarbon vapors located below the separator.
These are problems of the prior art which the present invention seeks to solve in a simple manner that is easy to implement.